1. Field of the Invention
The present invention relates to an adaptive equalizer used principally for a receiver for digital radio communication involving fading.
2. Description of Related Art
In a radio communication receiver, radio waves of a plurality of paths with different propagation times are generally received simultaneously due to influences of multi-path fading. For this reason, delay distortion occurs in a received signal and its bit error rate characteristic deteriorates. This delay distortion increases as the symbol rate increases and influences of delay distortion can no longer be ignored in high-speed digital mobile communication devices in the future. Therefore, eliminating delay distortion is becoming indispensable for the digital radio communication receiver.
An adaptive equalizer is conventionally incorporated in a receiver which carries out high-speed transmission as typical means for estimating the delay distortion and is also incorporated in a cellular phone, etc., based on GSM which is a digital cellular phone standard in Europe in recent years.
When equalization is performed on a transmission path in which multi-path fading occurs, transmission data having a training period is used. That is, transmission data is delimited in slot units and a training period is provided at the start of each slot. In this training period, default data called a “training sequence”, that is, a transmission signal with a default waveform is inserted. An adaptive equalizer mounted in a receiver makes a comparison with the default data during the training period and estimates the delay distortion. Then, the adaptive equalizer is designed to make a decision on data based on the estimated delay distortion in the part of the slot other than the training period. With reference to FIG. 1, an overview of a conventional adaptive equalizer will be explained below.
FIG. 1 is a block diagram showing a configuration example of a conventional adaptive equalizer. The adaptive equalizer shown in FIG. 1 is basically constructed of a first correlator 1, a second correlator 2 and a matrix operator 3.
The first correlator 1 receives a received signal converted to a baseband signal and a training sequence which is a signal out of the received signal known to the receiver as inputs, calculates a cross-correlation between the two signals and outputs a first correlation value calculated to the matrix operator 3. Here, a correlation sample interval τ is expressed as τ=T/K, where T is a symbol period and K is an oversampling rate. Furthermore, the correlation window size is L (L≧1).
The second correlator 2 calculates and stores an inverse matrix of an auto-correlation matrix between the training sequences. Or it calculates and stores an inverse matrix of the auto-correlation matrix beforehand. The calculation result of the second correlator 2 (second correlation value) is output to the matrix operator 3.
Using the output of the first correlator 1 (first correlation value) as a row vector and the output of the second correlator 2 (second correlation value) as a matrix, the matrix operator 3 multiplies this row vector by the matrix and outputs transmission path channel coefficients (c1 to cM) to a reception processing system.
The operation of the conventional adaptive equalizer in the above described configuration will be explained. A transmission path channel impulse response is updated from an input signal based on Expression (1) below:CT=(Σk=[t−L+1, t]XkAkT)×(Σk=[t−L+1, t]AkAkT)−1  (1)In Expression (1) , the first term on the right-hand side denotes the output of the first correlator 1 (cross-correlation between the received signal and training sequence) and the second term on the right-hand side denotes the output of the second correlator 2 (auto-correlation inverse matrix between the training sequences).
In Expression (1) , superscript T denotes a matrix transposition operation. Furthermore, a variable Xk with subscript k denotes a sampling value located at time t=kτ in the slot. L denotes a window size. Furthermore, a variable Ak with subscript k denotes each value at time t=kτ and is expressed by Expression (2):Ak=(ak, ak−1, . . . , ak−M+1)T  (2)In Expression (2) , ak denotes a known symbol located at time t=kτ in the slot.
A transmission path channel impulse response C in Expression (1) is expressed according to Expression (3) using coefficient cn of the transmission path channel impulse response estimated by the adaptive equalizer:C=(c1, c2, . . . , cM)T  (3)
However, the conventional adaptive equalizer requires matrix multiplication between 1×M matrix and M×M matrix and requires M2 complex multiplications and M2 complex additions. As a result, the amount of calculation increases in proportion to the square of the number of coefficients M of the transmission path channel impulse response calculated, and therefore there is a problem that power consumption increases.